Pictures and buttons can be in a stack's background, shared by every card in the stack, or they can be on individual cards.
If you put a picture or button in the background, you can change every card that shares that background by editing the picture or button in just one place. Putting a picture or button on an individual card lets you customize the card.
Choose Background from the Edit menu before you create a field, button, or picture you want to appear on every card that shares that background.